Masks used in lithography systems may be inspected by comparing light transmitted in the patterned (or “line”) region of the mask to the light transmitted in the non-patterned (or “space”) region. The defect detection sensitivity of the inspection system may depend on the difference in contrast between light transmitted in the two regions. Conventional transmissive optical masks may exhibit high inspection contrast since the light will either pass through the mask in the space region or will be blocked by the line region.
Low inspection contrast may present a problem in the inspection of reflective masks. Light absorber materials in the patterned region may typically reflect about 25% to about 45% of incident deep ultraviolet (DUV) light used for inspection purposes. The particular wavelength of the DUV inspection light is primarily determined by the inspection sensitivity requirement, typically in a range of about 150 nm to about 250 nm for extreme ultraviolet lithography (EUVL) mask inspection. The reflector region (e.g., the non-patterned region) may typically reflect about 60% of the DUV inspection light. As a result, the signal contrast between the patterned and non-patterned regions may be relatively low. In addition, there may be interference effects between the reflected light in the two regions due to the step height of the absorber. This interference effect may cause reflectance oscillation near the absorber edges, which may reduce the defect inspection sensitivity.